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Full text of "Text Book Of Mechanical Engineering"

Solid and Fluid Friction.

557

As there was probably considerable abrasion in these results,
it is doubtful whether they should be accepted, further than
generally, for pure friction. The second column shews that the
wheels should not be allowed to skid when stopping the train.

If surfaces are thoroughly lubricated the frictional resistance
is of a ' mixed' kind, being neither solid nor fluid. The following
comparison is useful:

COMPARISON OF THE LAWS OF SOLID AND FLUID FRICTION.

Fluid friction (gas or liquid) is:

1.  Independent of pressure.

2.  Directly as wetted surface.

3.  Directly as v at creeping velocities.

as iP at moderate velocities,
as zr> at high velocities.

Solid friction is :

1.  Directly as pressure.

2.  Independent of surface.

3.  Independent of velocity

(at low velocities).

The Friction of a Journal Bearing was investigated by
Beauchamp Tower for the Institute of Mechanical Engineers. The
load was carried on one braSs only, a top one, and the journal
ran in an oil bath. The coefficient varied with the lubricant.
With oil-bath lubrication Fn was independent of pressure, and

p, a -p.    In terms of velocity,         __

*-<-

where c varies with the lubricant.   Thus, when v = 4 and/ = 300.

Lubricant.
	c
	/*
	Lubricant.
	C    '
	/*

Olive oil ......
	280
	'OOIO2
	Sperm oil .
	104
	'00129


	
	
	
	
	

Lard oil   ......
	281
	'OOlSy
	Rape oil...
	"212
	"00141


	
	
	
	
	

Mineral grease
	43i
	00287
	Mineral oil
	-276
	00184

With syphon lubrication p = -^    where ^ = 2*02 for rape oil,

and with pad lubrication /* = *oi for rape oil.   The bearing seized
when/ rose above 600 Ibs.    (See Appendix //.,/. 870.)